Various blowing agents, including hydrocarbons among others, are often only partially soluble, if not completely insoluble, in many polyols used to manufacture rigid polyurethane foams. This is believed to be due to the non-polar hydrophobic characteristic of hydrocarbons. The insolubility or poor shelf life of hydrocarbon-polyol mixtures has, to date, restricted one against storing batches of polyol and hydrocarbon based blowing agent mixtures for use at a later time. Due to the poor solubility of various hydrocarbon based blowing agents in polyols, they must be added to the polyols under constant agitation and immediately before dispensing the foaming ingredients through a mixhead. The insolubility of various hydrocarbon based blowing agents also tends to lead to larger, coarser, or uneven cell structures in a polyurethane foam. As is well known, the thermal conductivity of a foam generally increases with a poor cell structure. Therefore, it has been critical that the blowing agent(s) employed be uniformly dispersed under constant agitation throughout the polyol mixture immediately prior to foaming in order to obtain a rigid polyurethane foam having the desired thermal insulation values.
In U.S. Pat. No. 5,391,317, Smits sought to manufacture a foam having both good dimensional stability and thermal insulation using hydrocarbons as blowing agents. This reference taught the use of a particular mixture of C.sub.5-6 alicyclic alkanes, isopentane and n-pentane blowing agents in particular molar percents, in combination with a polyol mixture made up of an aromatic initiated polyether polyol, an aromatic polyester polyol, and a different amine initiated polyether polyol. As the aromatic initiated polyether polyol, Smits suggested using an alkylene oxide adduct of a phenol-formaldehyde resin. The particular mixture of alicyclic and isomeric aliphatic alkane blowing agents is taught by Smits as producing a foam having good thermal insulation values.
The problem of obtaining a closed cell rigid polyurethane foam having both good dimensional stability and thermal insulation at low densities was also discussed in "An Insight Into The Characteristics of a Nucleation Catalyst in HCFC-Free Rigid Foam Systems" by Yoshimura et al. This publication reported the results of evaluations on a host of catalysts used in a standard polyurethane formulation to test the effects of each catalyst on the thermal insulation and dimensional stability of the foam. The authors noted that the solubility of cyclopentane in the polyol composition was reduced by increasing the blending ratio of aromatic amine-based polyols. Furthermore, not only did the authors note that the solubility of cyclopentane in the polyols was reduced as the aliphatic amine-initiated polyether polyol content was reduced and the aromatic amine-initiated polyether polyol was increased, but also noted that no significant effect in thermal conductivity was observed when the aromatic amine-initiated polyether polyol content was increased.